This invention concerns a method of preparing a solution effective in nitrosating nitrosatable aromatic compounds in an acidic, aqueous solution by means of the condensed gaseous reaction products of an alkali metal nitrite and a strong mineral acid.
It is known that sodium nitrite can be reacted with a strong acid in an aqueous solution to produce a solution effective in nitrosating aromatic compounds present in the solution. See, for example, U.S. Pat. No. 3,051,750 and No. 3,519,693. It is also taught in U.S. Pat. No. 3,051,750 that such a reaction must be carried out at temperatures in a range of about 0.degree. C. to 10.degree. C. to prevent the evolution of nitrogen oxides which might react with organic reactants to produce undesirable organic by-products. U.S. Pat. No. 2,225,357 discloses that the undesirable nitrogenous fumes can be removed from the reaction chamber by sweeping the atmosphere above the reaction solution with an inert gas.
It is known from U.S. Pat. No. 3,510,527 that nitrous acid, a nitrosating agent, is produced in aqueous nitric acid by addition of specific oxides of nitrogen at a temperature in the range of 0.degree. C. to 10.degree. C. Further, it is taught in this same patent that the nitrous acid can be decomposed to nitric acid and nitric oxide at a temperature in the range of 15.degree. C. to 30.degree. C. and subsequently the nitric oxide can be reformed as nitrous acid in an aqueous solution of nitric acid at a temperature in the range of 0.degree. C. to 10.degree. C.
It is generally desirable to produce a nitrosating agent by a method which does not introduce substantial amounts of alkali metal ions into the nitrosating solution. Repeated recycling of the mother liquor from a nitrosation process employing an alkali metal nitrite produces an excessive concentration of alkali metal ions which create a waste disposal problem complicated by the combination of inorganic ions with the residual organic compounds from the nitrosation. The use of oxides of nitrogen to produce a nitrosating solution does not introduce alkali metal ions into the nitrosating solution. However, the alkali metal nitrite, particularly sodium nitrite, is a less dangerous and a more economical reagent than are any of the oxides of nitrogen.
Nitrosation of a monosubstituted phenylic compound with subsequent oxidation of the nitroso group to a nitrate is a common synthetic method for preparing many disubstituted, para-nitrated phenylic compounds obtained in only low yields from the direct nitration of the related monosubstituted phenylic compounds such as phenol. It is, therefore, desirable to achieve high para-substitution in the nitrosation of monosubstituted phenylic compounds.
It is an object of this invention to prepare from relatively inexpensive reagents a solution of a nitrosating agent substantially free of alkali metal ions.
It is also an object of this invention to segregate in an inorganic solution exclusive of organic components the alkali metal salts produced from the reaction of the mineral acids and the alkali metal nitrite.
It is a further object of this invention to effect nitrosation of a monosubstituted phenylic compound with an unexpectedly high ratio of para- to ortho-substitution.